The present invention relates to a safety wheel for a subway coach bogie or the like, running on pneumatic tires, to a method for manufacturing such a wheel, and to a subway coach bogie or the like, running on pneumatic tires, equipped with such wheels.
A subway coach running on pneumatic tires conventionally comprises two bogies, each of which is fitted with pneumatic tires for rolling, disposed at the lateral ends of the chassis of this bogie. Between this chassis and each tire there is interposed a so-called safety wheel, coaxial to the tire and made of steel.
Each safety wheel presents a peripheral portion forming rim, comprising a braking portion or table, extending radially outwardly by a guiding portion or flange. When the subway is operating normally, the bogie rolls on its pneumatic tires, the safety wheels simply performing a function of guiding via their flange. The coach is braked both by current recuperation and mechanically. To that end, the braking table of the safety wheel is subjected to a pressure exerted by the sole of a braking member fast with the chassis. Moreover, in the event of puncture of the pneumatic tire, the safety wheel comes into contact with a rail, via the braking table, which ensures rolling of the coach.
These safety wheels are conventionally made of carbon steel according to type R2 of standard UIC 812-3. The method of producing them consists firstly in heating the wheel above the temperature of austenitization of the steel which consitutes it, i.e. about 890xc2x0. The wheel thus heated is subjected to a so-called standardized treatment, consisting in allowing it to cool slowly, without outside thermal action. The wheel thus obtained presents a homogeneous hardness of about 700 to 750 MPa, viz. between 195 and 220 on the Brinell scale.
However, this type of safety wheel presents certain drawbacks. In effect, it has been observed that their braking table is subject to rapid wear, this limiting their use to about 100,000 km in the case of mechanical braking. It is therefore necessary to change these wheels frequently, which leads to additional costs.
In order to overcome the different drawbacks of the prior art set forth hereinabove, the invention proposes a safety wheel which, while ensuring a reliable guiding of the bogie which is equipped therewith, has a longer lifetime than the wheels of the prior art.
To that end, the invention relates to a safety wheel for subway coach bogie, or the like, running on pneumatic tires, adapted to be interposed between a chassis of the bogie and a corresponding tire, said wheel presenting an outer peripheral zone comprising a braking portion adapted to receive the action of a mechanical braking member, as well as a guiding portion projecting radially outwardly from the braking portion, characterized in that the hardness of the braking portion is much greater than the hardness of the guiding portion.
Within the framework of the invention, the hardnesses of two members are compared from results of conventional tests for measuring the hardness of metals, such as for example the Brinell or Vickers methods.
According to other characteristics of the invention:
on the Brinell scale, the hardness of the braking portion is included between 293 and 311, while the hardness of the guiding portion is included between 195 and 220;
the wheel is made of carbon steel, in accordance with type R2 or type R8 of standard UIC 812-3;
said carbon steel includes, by weight, up to 0.700% of silicon, up to 0.120% of molybdenum and up to 0.400% of chromium.
The invention also relates to a method for producing a safety wheel for subway coach bogie, or the like, running on pneumatic tires, adapted to be interposed between a chassis of the bogie and a corresponding pneumatic tire, in which a steel blank is taken above the temperature of austenitic transformation of said steel, said blank presenting the shape of the definitive wheel and comprising an outer peripheral zone comprising a braking portion, adapted to receive the action of a mechanical braking member, as well as a guiding portion projecting radially outwardly from the braking portion, characterized in that it comprises the following steps of:
effecting a selective tempering of the braking portion of the blank taken above the temperature of austenitic transformation of said steel, without subjecting the guiding portion to this tempering, then
heating at least the tempered braking portion in order to effect an operation of annealing of this braking portion, with the result that the hardness of the braking portion of the definitive wheel is clearly greater than the hardness of the guiding portion of this wheel.
According to other characteristics of the invention:
a selective tempering of the braking portion is effected for a duration of between 5 and 15 minutes, so as to take this braking portion to a temperature of between 150 and 250xc2x0 C.;
the step of selective tempering of the braking portion comprises the following steps of: disposing the blank taken to above the temperature of austenitic transformation of said steel, in substantially horizontal manner, its guiding portion being placed above its braking portion, then spraying the braking portion with a cooling liquid;
at least the tempered braking portion is heated to a temperature of between 850 and 900xc2x0 C., for a duration of between 2 and 3 hours;
the whole of the blank is heated, after having effected tempering of the braking portion;
said blank is made of a carbon steel in accordance with type R2 or type R8 of standard UIC 812-3;
said carbon steel comprises, by weight, up to 0.700% of silicon, up to 0.120% of molybdenum and up to 0.400% of chromium.
Finally, the invention relates to a subway coach bogie, or the like, running on pneumatic tires, comprising pneumatic tires, a chassis resting thereon and safety wheels of which each is interposed between the chassis and a corresponding tire, characterized in that each safety wheel is such as described hereinabove.